Test for compile warnings

[straitm]

Along with running cppcheck, we should test for plain old compiler warnings. There shouldn't be any. This is a little tricky because different versions of gcc (to say nothing of different compilers) give different warnings. So the person who submitted the PR may not have gotten warnings while our test does. Either we can exclude common causes of this as we find them, or we can just tell the submitters what to do to fix the warnings they don't see.